1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a HDD with compensation for track misregistration (TMR).
2. Description of the Related Art
HDDs typically have disks with data tracks with a fixed track spacing or pitch that is set during manufacturing and cannot be changed during the life of the HDD. The positioning of the read/write heads to the data tracks is accomplished by servo tracks that have angularly-spaced servo sectors that contain head positioning information, typically position error signal (PES) blocks or bursts. The servo sectors extend radially across the data tracks. As the disk rotates the read head detects the PES blocks, which are decoded into a PES that is used by the servo control system to maintain the head on the desired data track. The servo tracks also have a fixed track pitch that is set during manufacturing and that may be different from the fixed track pitch of the data tracks.
A read head following a particular position with respect to a servo track during writing may be subject to various disturbances, such as internal and external vibration. This may cause the write head to be off track when the data sectors are written in the data tracks. During readback this results in a misalignment between the read head position and the position where the data track was actually written. This is true even if the read head is in its ideal position with respect to the servo track. This discrepancy between written position and the read head positioning during readback is referred to as track misregistration (TMR). TMR results in readback data errors and is a major detractor from achieving the highest possible areal densities and high throughput in modern HDDs. The data errors may possibly be recoverable in a re-read of the data, but this increases the time to retrieve the data; or they may not be recoverable, resulting in hard errors.
HDDS may use “shingle writing”, also called “shingled recording” or “shingled magnetic recording” (SMR). SMR HDDs are well-known, for example as described in U.S. Pat. No. 6,185,063 B1 and U.S. Pat. No. 6,967,810 B2. In SMR, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the shingled data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. The narrower shingled data tracks thus allow for increased data density. Because the number of readback errors due to TMR increases with data track “squeeze”, i.e., closer spacing of the data tracks with decreasing track pitch, TMR is also a problem with SMR HDDs, which are designed to have very narrow data tracks.
What is needed is a magnetic recording disk drive that compensates for TMR during readback.